Giant cell arteritis (GCA) is a sight-threatening systemic vasculitis with an immune-mediated pathogenesis. Vascular lesions, composed of activated T cells and macrophages, induce progressive, insidious vascular occlusion of medium-size arteries, resulting in tissue ischemia, such as ischemic optic neuropathy. Our long-term objective has been to unravel the mechanisms by which lymphocytes and macrophages are recruited to the vessel wall, differentiate into specialized effector cells, and promote GCA. During the last four years, we have made progress towards these goals by dissecting macrophage effector pathways that lead to arterial injury and the artery's response-to-injury program. Most importantly, we have provided evidence that the primary defect in GCA is a breach of T-cell tolerance and that T-cell activation occurs in the adventitia where dendritic cells (DC) initiate and maintain T-cell stimulation. We hypothesize that adventitial DC are key players in generating immunological privilege for the artery and that breakdown of this immune protection causes GCA. This hypothesis includes that structural and cellular components of the adventitia create the unique target-tissue susceptibility and age-dependence of GCA. Experiments to test this hypothesis will use a temporal artery-NOD/LtSz-Rag1 (tm1 Mom) mouse chimera model. The specific aims of this proposal are to: 1) Examine the heterogeneity of adventitial DC and vasa vasorum networks in different vascular beds and at different ages. Three-dimensional rendering of capillary networks in the adventitia and their relationship to DC indigenous to the adventitia will be examined by microcomputed-CT to define a model for the targeting of GCA to extracranial arteries in elderly individuals. 2) Determine the mechanisms of immune tolerance promoted by adventitial DC. These experiments will use adoptively transferred T cells in mouse chimeras. 3) Investigate the functional profile of adventitial DC in GCA and polymyalgia rheumatica (PMR), a forme fruste of GCA. Disease-relevant defects in DC function will be identified by gene expression profiling. 4) Explore the mechanisms of inappropriate arrest and premature differentiation of DC in PMR and GCA arteries. These studies will focus on pathways regulating the life cycle of DC and deviations leading to vasculitis. 5) Explore the therapeutic implications of modulating DC function in GCA. We will proceed with two approaches, TNF-alpha inhibitors and tolerogenic DC, in the attempt to disrupt persistent DC and T-cell activation in GCA. These experiments will help us understand the immunobiology of healthy and inflamed arteries, advance our understanding of the events initiating vasculitis, and may provide new avenues for therapeutic intervention.